Low power solid state oscillators are widely employed in electronic systems used in such diverse fields as communications, control and instrumentation. In many of these systems, the frequency of oscillation is required to be electrically controllable. Examples of such applications are the common automatic frequency control (AFC) circuit of an FM receiver and the ubiquitous phase-locked loop (PLL) circuit found in FM demodulators, frequency synthesizers, and countless other electronic systems.
BJTs have frequently been used in electrically tuned oscillators as the active (i.e., gain producing) element. Such circuits have generally obtained electical control of the oscillation frequency by employing a reverse-biased pn junction as a capacitive element in an LC tank circuit. This reverse-biased pn junction may simply be the base-collector junction of the BJT itself, or it may be a separate reverse-biased diode known as a "varactor". Some circuits utilize both a varactor diode and the BJT's base-collector junction in parallel in the frequency determining network.
Increasing the voltage across a reverse-biased pn junction causes the junction's depletion region (a narrow region near the junction that is devoid of mobile charge carriers) to widen. This widening, in turn, results in a decrease in the capacitance of the junction; the so-called junction depletion capacitance. Oscillator circuits that rely upon variations in junction depletion capacitance to vary oscillation frequency are inherently voltage controlled oscillator (VCO) circuits. Further, they are characterized by having a positive frequency-voltage characteristic; that is, an increase in junction bias voltage results in a corresponding increase in the frequency of oscillation. Since depletion capacitance is a nonlinear function of junction voltage however, such VCO circuits have highly nonlinear frequency-voltage characteristics. In addition, they may require relatively complex biasing networks.